Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for spark-ignited gasoline (e.g., non-gaseous) engines typically monitor the release of carbon monoxide, nitrogen oxides (NOx), and unburned hydrocarbons (UHC). Catalytic converters (e.g., oxidation catalysts) implemented in an exhaust gas aftertreatment system have been used to eliminate many of the regulated pollutants present in exhaust gas generated from gasoline powered engines. For example, some known three-way catalysts include carefully selected catalytic material formulations to specifically oxidize carbon monoxide and unburned hydrocarbons, and reduce nitrogen oxides to less harmful components, present in the exhaust gas. Conventional three-way catalysts are designed to oxidize or reduce such pollutants more efficiently for engines running above the stoichiometric air-to-fuel ratio (i.e., rich conditions).
Recently, due at least in part to high crude oil prices, environmental concerns, and future fuel availability, many internal combustion engine designers have looked to at least partially replace crude oil fossil fuels, e.g., gasoline and diesel, with so-called alternative fuels for powering internal combustions engines. Desirably, by replacing or reducing the use of fossil fuels with alternative fuels, the cost of fueling internal combustion engines is decreased, harmful environmental pollutants are decreased, and/or the future availability of fuels is increased. Known alternative fuels include gaseous fuels or fuels with gaseous hydrocarbons, such as, for example, natural gas, petroleum gas (propane), and hydrogen. The combustion byproducts present in exhaust gas generated by spark-ignited gaseous-powered engines are similar to those present in exhaust gas generated by spark-ignited non-gaseous-powered engines. Accordingly, conventional gaseous-powered engine systems utilize the same oxidation catalysts found in non-gaseous-powered engine systems to oxidize the regulated pollutants generated by gaseous-powered engines.
However, gaseous-powered engines also generate exhaust gas with relatively large amounts of presently unregulated pollutants, such as methane. Traditionally, gaseous-powered engines are operated at rich air-to-fuel ratios (e.g., richer than stoichiometric) in order to reduce oxygen concentrations within the exhaust gas, and thus the formation of carbon monoxide and nitrogen oxides. Operating under such rich air-to-fuel ratios consequently generates very high levels of unburned hydrocarbons, such as methane. Conventional gaseous-powered engine systems do not include oxidation catalysts capable of oxidizing methane. Accordingly, gaseous-powered engine systems allow large amounts methane to escape into the atmosphere.
Additionally, operating a gaseous-powered engine under stoichiometric or richer air-to-fuel ratios results in a relatively low brake thermal efficiency of the engine. Moreover, operating at such air-to-fuel ratios causes high combustion temperatures, which result in high component temperatures in the engine, and the necessity to reduce output power to avoid component failure. However, in view of the premium placed on satisfying exhaust emissions regulations, conventional gaseous-powered engines are designed to meet exhaust emissions regulations at the expense of thermal efficiency and power density.
Further, as recognized by the inventors, some internal combustion engine systems that employ exhaust gas recirculation (EGR) strategies often suffer from the formation of harmful condensates within the air and charge air intake line. For example, the presence of certain emissions in the recirculated exhaust (e.g., CO, CO2, NOx, and UHC) can cause the formation of aggressive acids in the condensate within the intake circuit of the engine should charge temperature fall below the dew point at any location within the intake system. CO and CO2 may result in carbonic acid forming in the condensate within the intake system and NOx may result in the formation of nitric acid within the condensate. Of these two acids, nitric acid is more aggressive and has the greater potential to negatively affect the service life of the components of the intake system.